The present invention solves a critical problem in optical communication systems. The channel waveguide horn structures formed in accordance with the present invention not only improve the coupling efficiency between the channel waveguide and a single mode optical fiber by a significant amount, but also reduce the tolerance requirements for the coupling between such single mode optical fibers and channel waveguides. The present invention makes end-butt coupling techniques feasible for field operation.
The coupling between single-mode optical fiber and channel waveguides is a critical aspect of the realization of many circuits. Of all the coupling schemes proposed in the literature, end-butt coupling is the most straightforward and pracitcal, particularly when low index glass fiber (N=1.5) is to be coupled to high index LiNbO.sub.3 channel waveguide (N=2.20). High coupling efficiency is expected when the mode intensity profile of the two waveguides are properly matched and when the optical fiber is aligned precisely to the channel waveguide. The Fresnel reflection loss at waveguide termination can be reduced by adding a bit of index matching epoxy or anti-reflection coating at the waveguide ends. Usually, the single-mode channel waveguides used in optical circuits require shallow waveguide depths to obtain a large interaction between the guided optical field and surface accoustic or electric field. The typical cross-section of a Ti-diffused LiNbO.sub.3 channel waveguide is about 4 by 2 micrometers; however, single-mode optical fibers have relatively larger core diamenters, ranging from 5 to 15 micrometers. To assure a good mode matching in optical field, one can use either a small core fiber or the three-dimensional horn structure of the present invention at the end of the channel waveguide. Using small-core fibers in an optical system is not practical because of a severe coupling tolerance requirement. In contrast, the combination of the present three-dimensional horn structure and a relatively large core fiber will make the tolerance less critical.
In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a method for for producing enhanced diffusion so that an optical waveguide may have varying cross-sections along its length.
It is therefore an object of the present invention to provide a method to produce a three-dimensional horn structure at the end of a channel waveguide which is suitable for butt coupling to a single mode fiber optic.
It is a further object of the invention to provide a method for fabricating such a three-dimensional horn structure.
These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the more detailed description below taken in conjunction with the accompanying drawings wherein like reference characters refer to like parts throughout.